.\" -*- nroff -*-
.\" Copyright 2013 Los Alamos National Security, LLC. All rights reserved.
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Unpack_external 3 "Jun 10, 2020" "4.0.4" "Open MPI"

.SH NAME
\fBMPI_Unpack_external\fP \- Reads data from a portable format

.SH SYNTAX
.ft R

.SH C Syntax
.nf
#include <mpi.h>
int MPI_Unpack_external(const char \fIdatarep\fP[], const void *\fIinbuf\fP,
	MPI_Aint \fIinsize\fP, MPI_Aint *\fIposition\fP,
	void *\fIoutbuf\fP, int \fIoutcount\fP,
	MPI_Datatype \fIdatatype\fP)

.fi
.SH INPUT PARAMETERS
.ft R
.TP 1i
datarep
Data Representation (string).
.ft R
.TP 1i
inbuf
Input buffer start (choice).
.TP 1i
insize
Size of input buffer, in bytes (integer).
.TP 1i
outcount
Number of items to be unpacked (integer).
.TP 1i
datatype
Datatype of each output data item (handle).

.SH INPUT/OUTPUT PARAMETER
.ft R
.TP 1i
position
Current position in buffer, in bytes (integer).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
outbuf
Output buffer start (choice).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
MPI_Unpack_external unpacks data from the external32 format, a
universal data representation defined by the MPI Forum. This format is
useful for exchanging data between MPI implementations, or when
writing data to a file.
.sp
The input buffer is a contiguous storage area pointed to by
\fIinbuf\fP containing \fIinsize\fP bytes. The output buffer can be
any communication buffer allowed in MPI_Recv, and is specified by
\fIoutbuf\fP, \fIoutcount\fP, and \fIdatatype\fP.
.sp
The input value of \fIposition\fP is the first position in \fIinbuf\fP
to be read for unpacking (measured in bytes, not elements, relative to
the start of the buffer). When the function returns, \fIposition\fP is
incremented by the size of the packed message, so that it points to
the first location in \fIinbuf\fP following the message that was
unpacked. This way it may be used as input to a subsequent call to
MPI_Unpack_external.

.SH NOTES
.ft R
Note the difference between MPI_Recv and MPI_Unpack_external: In
MPI_Recv, the \fIcount\fP argument specifies the maximum number of
items that can be received. In MPI_Unpack_external, the \fIoutcount\fP
argument specifies the actual number of items that are to be
unpacked. With a regular receive operation, the incoming message size
determines the number of components that will be received. With
MPI_Unpack_external, it is up to the user to specify how many
components to unpack, since the user may wish to unpack the received
message multiple times into various buffers.
.sp
To understand the behavior of pack and unpack, it is convenient to
think of the data part of a message as being the sequence obtained by
concatenating the successive values sent in that message. The pack
operation stores this sequence in the buffer space, as if sending the
message to that buffer. The unpack operation retrieves this sequence
from buffer space, as if receiving a message from that buffer. (It is
helpful to think of internal Fortran files or sscanf in C for a
similar function.)
.sp
Several messages can be successively packed into one packing
unit. This is effected by several successive related calls to
MPI_Pack_external, where the first call provides \fIposition\fP=0,
and each successive call inputs the value of \fIposition\fP that was
output by the previous call, along with the same values for
\fIoutbuf\fP and \fIoutcount\fP. This packing unit now contains the
equivalent information that would have been stored in a message by one
send call with a send buffer that is the "concatenation" of the
individual send buffers.
.sp
A packing unit can be sent using type MPI_BYTE. Any point-to-point
or collective communication function can be used to move the sequence
of bytes that forms the packing unit from one process to another. This
packing unit can now be received using any receive operation, with any
datatype: The type-matching rules are relaxed for messages sent with
type MPI_BYTE.
.sp
A packing unit can be unpacked into several successive messages. This
is effected by several successive related calls to
MPI_Unpack_external, where the first call provides \fIposition\fP=0,
and each successive call inputs the value of position that was output
by the previous call, and the same values for \fIinbuf\fP and
\fIinsize\fP.
.sp
The concatenation of two packing units is not necessarily a packing
unit; nor is a substring of a packing unit necessarily a packing
unit. Thus, one cannot concatenate two packing units and then unpack
the result as one packing unit; nor can one unpack a substring of a
packing unit as a separate packing unit. Each packing unit that was
created by a related sequence of pack calls must be unpacked as a unit
by a sequence of related unpack calls.

.SH ERRORS
.ft R
Almost all MPI routines return an error value; C routines as
the value of the function and Fortran routines in the last argument. C++
functions do not return errors. If the default error handler is set to
MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
will be used to throw an MPI::Exception object.
.sp
Before the error value is returned, the current MPI error handler is
called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with
MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN
may be used to cause error values to be returned. Note that MPI does not
guarantee that an MPI program can continue past an error.
.sp
See the MPI man page for a full list of MPI error codes.

.SH SEE ALSO
.ft R
.nf
MPI_Pack_external
MPI_Pack_external_size
MPI_Recv
sscanf(3C)

